Induction heating technology is well known and in wide spread use in industrial and commercial applications. One of the advantages of induction heating is the xe2x80x9cnon-contactxe2x80x9d aspect of the technology. In particular, an induction heater uses magnetic fields to energize a heating element formed of a suitable radiation-sensitive material. The magnetic field generator need not be in contact with the heating element or even the item which is itself to be elevated in temperature. This arrangement makes induction heating a wise choice in applications where the heated item must easily be moved. These include industrial applications such as assembly lines or branding irons, as well as commercial food and plate warming. Other applications involve containers for take out food, such as pizza delivery bags, for example. These containers have typically been made with an external temperature indicator and a heating element heated by an AC source. These containers include an AC cord which can potentially entangle a user, creating safety issues when the container is transported.
There is a problem however with some of these applications. A plate warmer for example, needs to maintain the temperature of the plate below some defined allowable value. This is especially important if the plate is to be handled by a person, or if the plate is constructed of a plastic/metal composite.
One way to control the final temperature of the plate can be to apply the induction heating to the plate for a specific time duration. This method can provide poor results, unless the temperature of the plates was controlled before the start of the heating process. For example, if the same plate was exposed to an induction heater twice in a row, one time right after another, the plate can rise to a much higher temperature.
Another method of controlling the final temperature of the plate uses an external temperature sensor to measure the temperature of the plate before, and/or during the induction heating process. The sensor can be a xe2x80x9ccontactxe2x80x9d or xe2x80x9cnon-contactxe2x80x9d type. The xe2x80x9ccontactxe2x80x9d type of temperature measurement spoils the inherent xe2x80x9cnon-contactxe2x80x9d nature of the induction heating process. Additionally, it can be difficult to get the sensor to contact the correct surface of the heating element while providing a reliable, robust design. The xe2x80x9cnon-contactxe2x80x9d type of temperature measurement is better, but more costly.
A completely different solution might involve a specially formulated metal heating element that only xe2x80x9ccouplesxe2x80x9d (i.e., allow currents to be induced) with the induction field if the temperature of the metal is below some pre-determined value. These metals have a Curie point that prevent the metal from overheating, even though the induction field is still present.
The problem with the above methods is that none provide the capability of temperature indication, status monitoring, or other electronic functions without a power supply within the container or a wired, physical connection between the container and an external heater. These methods also do not provide electronic functions after the heated item is removed from the induction heating device.
A solution to this problem is to place an induction-driven power supply within the electromagnetic field used to heat the heating element. The power supply can, for example, include an induction coil across which is induced a current. In an alternate embodiment, this can be provided by an opening or slot formed on the heating element, the opening having a first lead and a second lead, wherein the opening creates a voltage differential transferred to the first lead and the second lead.
The power supply is used to provide power to various electrical circuits which accompany the heating element. For example, these circuits may include a control system having a temperature sensor, a temperature indicator, and a communication link, such as an RF, light or sound link, which electronically controls the operation of induction source. The controller can communicate to the inductor, via the communication link, if more heating power is necessary and to indicate the desired temperature has been reached. The temperature indicator indicates when the element has reached an acceptable temperature and the unit is ready to be used.
Additionally, the circuits may include energy storage devices, such as rechargeable batteries or capacitors, which are charged while the device is subjected to the electromagnetic field during the induction heating process. These energy storage devices permit the circuit to continue operating even when the container is removed from the electromagnetic field source.
In the case of the controller, the stored energy permits the monitoring of the temperature of the heating element with status LEDs even after the device has been removed from the inductor.
The induction driven circuit and heating element are preferably used in conjunction with a container for heating of food items.
The electromagnetic field can be generated by a single induction source. The induction source can also include a plurality of induction sources. A first induction source and a second induction source can be utilized where the first induction source heats a heating element and the second induction source powers a circuit.